Preparation of metallic surfaces for cladding with comminuted metals and the products so clad



PREPARATHQN 9F METALLIC URFAES F6112 CLADDING WKTH COMMINUTED METALS AND THE PRQBUCTS S CLAD A.

Rolfe Pottherg, Baltimore, Md, assignor to Peen Plate, lino, Baitimore, Md, a corporation of Maryland No Drawing. Filed Nov. 15, B60, Ser. No. 69,297

13 Claims. Ci. 29-194) This invention relates to the'preparation of metallic surfaces for the application of coatings of other metals where these are applied in solid, cornminuted form to the prepared surface and productsso coated.

This application is a continuation in part of my pending application Serial No. 388,702, filed October 27, 1953.

In application, Serial No. 388,702, it was shown that thin films of copper applied by simple immersion treatments served greatly to facilitate the deposition and adhesion of metal coatings originaally supplied in the form.

of finely divided solid powder particles, usually in liquid environments such as those described in US. Patent 2,640,002 and British Patent 534,888, and under'conditions tending to densify and consolidate'layers of such particles into a dense, adherent film, usually by subjecting to agitation and impact in the presence of a surrounding, suspending mass of suitableimpacting particles.

It has since been found that the mechanism presumed effective in the former case, i.e., metallurgical compatibility or relative ease of Wetting of one metal by another, or mutual solid solubility, is not the only mechanism responsible for the effectiveness of such films as a preliminary surface treatment for such method of metal clading; but that another and in some cases more important mechanism is operative. This in turn has led to the discovery of other such preliminary metal films, some of, which are effective to an even greater degree than dedifferences and deposition efiiciencies in whatever liquid 7 solutions are used; if the solution potential of the base metal surface is lower than that of the metal powder particles. Other metals which may' be applied by immersion means are also effective, some of them to an even greater degree than others and, in many instances, certain metals having the immersion coating thereon will be receptive to the metal particles which'without the immersion coating will not be receptive to the metal particles.

thereof whereby they are substantiallythe sam'e'point to point; thus giving'the advantage that the surface of the thus coated base metal is'metallurgically uniform and themetal particles subsequently applied thereto accumulate and adhere with greaterv uniformity.

A still further object of the invention is to provide a coated object which, in addition to being more uniform, 1s superior-in corrosion resistance by virtue not only of added coating uniformity but which is also quantitatively superior in the sense of having a thicker coating as a re sult of the particular processing, as well as a coating which is qualitatively superior due not only to superior adhesion but also to favorable galvanic effects arising from the imposition of the thin, more noble metal film between the less noble base metal and metal cladding surrounding it. i e

It has been found by actual tests and examples hereinafter recited that when 'a piece of, a base metal or an article of a basemetal has applied thereto a thin coating of a metal having a lower solution potential than that of the metal particles tobe applied and that when the thus An object of the present invention is to so prepare a metallic surface by applying thereto a thin adherent coating of another metal having the unique property of caus:

ing finely divided particles ofa third metaltobe attracted and adhered thereto inpreparation for later steps to fur ther coalesce and density the particles.

Another object of the inventionis to prepare'the surface of the base metal by applying a thin immersion coating of a metal thereto having a lower solution potential than the base metal and then applying finely divided par ticles of a metal having a-higher solution potential than the immersion coating, to which immersion coatingthe particles will be caused to be attracted and adhered in preparation for whatever steps are taken simultaneously or later to consolidate or densify the. coating of the particles.

Another object of the invention is to overcome any unevenness of the surface characteristics of the base metal by the immersion coating, which produces an even coating over the base metal and changes the characteristics ".ferred to herein defines the thin coating which is applied coated base metal is placed in agitated relationship to a "slurry containing the metal particles in usually an acid or an acid salt solution, the metallic particles will tend tobe attracted to the coated surface of the metallic article and will be caused to accumulate upon and adhereto the a coated surface andalso to each other with tenacity so that to finally compact the particles against eachother and the surface of the immersion coating. By impacting 'media is meant a matrix material consisting of a mass of discrete solid granules usually much smaller than the articles or objects but much larger'than the metal of, the powder particles, inert to the particular chemical environments employedand serving the purpose of separating and suspending the objects from one another in the course of mechanical agitation, as well as to supply a multiplicity of randomly distributed impacts to the'surfaces being so coated. Such granules may be spherical or-irregulargand' be metallic, ceramic, mineral, vitreous or even consist of the objects themselves if these are small and numerous I enough.

However, when placed in an agiteclv slurry, the metallic articles are found to be relatively-adherently coated with undistorted particles of the metal coating powderin the absence of any impacting or other forces sufficient to affect final consolidation of the coating.

The preliminary, immersion or substrate coating redirectly to the metallic object and to which subsequently .is appliedaco'ating of the metal powder particles.

" I use'as an immersion coating. salts'of the following ,metals: cadmium, tin;lead, silver, copperand mercury.

The metal particles to be applied to the immersion coating are s'elected from the group: zinc, cadmium, tin, 1

lead, nickel, copper'and silver and alloys thereof, in gencopper.

:The invention inone: of it saspects, involves the process of treating the base metalito be coated in ionized solutions of salts of metals having afl lower solution potential than themetal of the base; Sincethe suitable metals can f V usually be easilygdepo'sited by well-known, immersion meanson the most commonly encountered, i.e., ferrous 3,1 4,448 Patentedtjani 5, ,J

I r eneaaae surfaces, I utilize a salt of a metal which has alower solution potential than the metal of the base; It is usually not necessary to resort to electrolytic means forthe deposition of the lower metals. In general, the lower the solution potential of the suitable metals recited the more readily the immersion coating is adhered to the surface of the base metal. For reasons recited in the original application and which apply equally as well in this application, copper is and remains a logical, highly effective and economical choice as the metal of the immersion coating; particularly where metals such as zinc and cadmium are to be deposited on ferrous basemetals.

Procedures for applying immersion films of the preliminary coating metals recited are well-known to the art and appear in standard reference texts available to the metal plating trades, for instances, as disclosed in, and may be considered as part hereof, the publication published by Finishing Publications, Inc. entitled Metal Finishing Guidebook Directory. Improved proprietary formulations and materials are also available.

The metal article may have various surface unevennesses, such as unevenness in its metallurgical characteristics, which characteristics may have been formed during the molding or working or heat treatment of the article. The present invention overcomes unevennesses from point to point on the surfaces; both of metalurgical characteristics and of relative solution potential; such as to affect the evenness with which the particle layer accumulates. The immersion coating itself has the same characteristics from point to point, thus making a uniform surface on which the particles are subsequently uniformly accumulated. The immersion coating so masks the metallic article that the particles being applied thereto are not affected by the base metal but are only affected by the immersion coating.

The thu coated articles are placed in a slurry, which slurry will contain the metallic particles in a, preferably acid, salt solution with or without an impacting medium. Thus slurry, together with the coated metallic articles may then be placed, for example, in a tumbling drum and the same rotated for a predetermined period of time. When the metallic articles are removed from the drum, it will be found that they have adhered to their surface practically all of the metallic particle-s. Especially if an impacting media is used in the drum and when the articles are removed therefrom, the metallic particles will be found to have been compacted and densified on the article and the article is now ready for the use to which it is to be put without further treatment. The type of coating which results when the final coating has been densified in the foregoing manner may generally be described as follows, following metallographic sectioning and chemical treatment:

A very thin, crystalilne deposit of the lower solution potential metal deposit fills microscopic pores and cavi ties in the base metal and can be seen to be sandwiched between the latter and a much thicker coating comprised of the original metal powder particles. A section of this final coating in turn, after appropriate chemical etching, can be seen to consist of the particles of the original coating metal perceptibly distorted in such a manner as usually substantially to eliminate all intersticial voids and in some cases to establish a metallurgical bond between the particle interfaces. This distortion is in a horizontal plane parallel to the surface to which the coating is applied and may vary from such distortion as is necessary simply to substantially eliminate all intersticial voids between such original particles, to'a point at which the particles are extremely distorted 'in a horizontal dimension with corresponding reduction in vertical dimension; resulting in a lamellar or stratified appearance. A further and often distinguishing characteristic of the particular type of coating which results, following resort to. the herein set forth expedients, is that traces of the metal of the immersion coating can bemetallographically discorned between particles of Thesetraces may be in the form of discrete -occlusions-of the original immersion coating metal between particles of the cladding metal or coating and a stronger afinity for one another than is' the case in the absence of the favorable substrate coatings claimed herein. that the substrate coating attracts the metal particles to the surface prior to actual physical contact therewith, and the particles are sufficiently strongly adhered to the substrate coatings and to each other that they strongly resist removal by the mechanical and hydraulic forces encountered in the course of the plating operation. Additionally, such tenacious, initial adherence 'makes less critical the choice of mechanical conditions necessary to. avoid chipping off of the coating at sharp edges prior to the time that a final coating has been fully metallurgically integrated and adhered. I

It is sometimes desirable to apply an intermediate immersion coating, in cases where the solution potential of the immersion coating desired is so much lower than that of the metal of the article itself that spongy and poorly adherent immersion coatings result, unless an immersion coating of intermediate solution potential is interposed.

he use of copper prior to application of silver or mercury is an example.

As an example in which differently treated surfaces have metallic particles applied thereto under otherwise identical conditions, but in physical separate apparatus, the following test was made:

Four pound batches of 1" diameter by hot rolled steel washers were used in which all were preliminarily thoroughly cleaned and all but one batch immersion coated with tin, lead, copper, silver and mercury respectively. The slurry in each case consisted of six pounds of 12 mesh fused aluminum oxide granules, 170 grams of fine, high purity condensed zinc dust and approximately 600 cc.s of water acidified with grams of zinc chloride. (In the one batch having an immersion. coating of silver thereon, an equivalent amount of hydrofluoric acid was used). Each batch was agitated 'in'a small, approximately 4000 cc.s tumbling mill for two hours at ambient temperature. After the treatment the washers were removed from each batch and washed and the thickness'of thezinc was measured on each and found to be as follows:

Thickness of zinc Such coating thicknesses are found to generally reflect the ditferentsolution potential with respect to zinc in the particular. electrolyte or acid or acid salt employed.

In another example, similar objects with dissimilar metal surfacing were plated together at the time in the same apparatus. Two lbs. of 1%" by 11 gauge iron roofing nails were divided intofour equal half pound quantities. One was'merely acid cleaned, and each of the three remaining half pound quantitiesreceived immersion coatings respectively of copper, silver and mercury. (For reasons the;final, coating metal adja- I cent to the interface of the two coatings and at some distance from the original surface Further, there is evidence toindicate' previously stated the latter two immersion coatings were superimposed upon intermediate immersion coatings of copper. Such immersion films deposited under normal conditions are extremely thin by comparison withdiscernible thicknesses recited herein and the extent 'to'which their thickness contributes to that of the final film can be disregarded.) The two pounds of nails were placed in a tumbling mill together with six of glass beads of sizes ranging between minus 25 mesh to plus 100 mesh, 85 grams of zinc dust and 600 cc.s of water acidified with seven cc.s of 180 sulphuric acid. The drum was rotated for 1 /2 hours and at the completion thereof the articles were observed to have the following amounts of plating thereon:

Thickness of zinc Surface: deposited inches Clean iron .0000

Immersion copper- .0015 Immersion silver .0047 Immersion mercury g I .008

1 No plate. I r

in the presence of a relatively large volume (55' soft poly-' ethylene granules, to minimize the possibility of inter-1 dust in 4000 cc. equipment. .In spite of the fact that test conditions were established to minimize impact and attrition adequate to affect the plastic deformation or swaging of the metal of the powder particles, it was observed that a hard, thick, relatively adherent (although almost comv pletely unpolished) coating resulted.

In a further example in similar equipment two separate,

two pound batches of such 1" washers were processed under otherwise identical conditions using minus 395 mesh brass powder, glass beads and a dilute citric acid solution. Half the quantities of washers in each test were merely cleaned and the other half surfaced respectively with im- The results in mersion coatings of mercury and silver. the two tests showed a continuous brass coating for the immersion surfaced washers and a discontinuous coating of the unsurfaced washers with a majority of the area of the substantially continuous were only approximately 1 powder particle thick, and in the third and fourth tests previously tin and zinc coated objects received no coating. (In the latter two cases the tin and zinc were mechanically applied.)

In another test one inch clean steel washers were placed in one drum and a similar amount, which were immersion coated with mercury, in another, each drum receiving six pounds of 12 mesh synthetic alumina granules, 1% citric acid and 100 grams minus 395 meshbrass powder. After 1 hour the initially unsurfaced steel washers had a film of brass so light as to be characterizable only as a color whereas the mercury coated washers had .0008" of brass thereon.

If copper powder is substituted for brass powder proportionate results are observed except that copper deposits with somewhat greater efficiency from similar solutions. The efficiency of the deposition of both copper and brass appears to be improved if sulphuric or phosphoric acid is substituted for the citric acid in those examples mentioned above.

In the depositing of nickel on steel, two tests were made using clean steel objects in one test and steel objects having an immersion coating of lead thereon. The slurry contained fine nickel powder, citric acid and small glass beads approximately three times the weight of the objects. The unsurfaced steel objects receiveda coating which could not be measured even by electromagnetic means but those immersion coated with lead received an apparently continuous coating of nickel.

To indicate the extent to which the particles tend to adhere to one another and to the base prior to, or in the absence of more drastic means of effecting their final consolidation into an integrated coating the following test was made. A small quantity of small nails was used It has been shown herein that the particles will adhere to 'metallic articles previously surfaced with herein recited coatings within a relatively short-time. It has been further shown that objects with an immersion coating of copper,.for example, agitated in the presence .of slurries of fine glass beads and zinc dust in the appropirate solutionsmayaccumulate on their surfaces the majority of all the dust present in their vicinity within a space of a few minutes; the dust being distributed uniformly over the surfaces of the objects and being, naturally, sufiiciently adherent tohave resisted the forces of attrition, mechanical and hydraulic, inherent in the conditions of operation. -Under these conditions the zinc dust is not naturally perceptibly affected and still resembles the original zinc dust in appearance. In such instances the matrix 1 of impacting particles have served simply to uniformly disperse and separate both objects and zinc dust particles].

Such

and provided no significant mechanical-action. coatingseven in this condition are sufficiently dense and they have a limited usefulness jafter drying the same at vmoderate temperatures. I I

Thus it will beseen from the examples thatrnetal surfaces having a-coating referred to herein of metal having:

a lower solution potential than the metal of the surface, and deposited from ionized solution, can be coated with layers of metallic powder particles of' a metal having a higher solution potential than that of the coating, and that the resulting such coating layer of particles is intrinsically more strongly adherent to such coated metal surface, prior to.or in the absence of other means to further density and consolidate such overlayer of particles. Further, it

has been shown that more than one means may be used to vfinally prepare such overlayer of powder particle for final usefulness. It has also been further shown that the majority of the metal powder particles are attracted and will adhere to the immersion coated article within -a relatively short time, sufiiciently strongly to resist removal under sometimes drastic conditions of mechanical and hydraulic scrubbing.

I claim:

1. Ina process for cladding metal article surfaces with article surfaces by immersing the metal article in a solution of salts of said preliminarily applied third metal.

3. The method as set forth in claim I in whichthe preliminarily applied coating is'selected from the'class of metals consisting of cadmium, tin, lead, copper, silver and mercury.

4. The method as set forth in claim l in which the solid particles are selected from the group of metals consisting of zinc, cadmium, tin, copper, lead, nickel, silver andthe alloys thereof.

5. In a process for cladding metal article surfaces with adhered, compacted multilayers of interfitting, solid and discrete mechanically distorted particles of another metal by immersing said surfaces in dynamically agitated, hydraulic suspensions of said metal particles in acidified solutions, the step of preliminarily applying directly onto said surface a relatively thin layer of a third metal having a lower solution potential than the metal of the metal powder particles.

6. An article having a base metal surface and a permanent metallic coating thereon, the coating consisting essentially of:

a relatively thin, ionized salt solution deposited crystalline coat of a first metal adhering directly to the base metal surface; and

a relatively thick coat of built up layers of mechanically distorted and closely interfitting discrete and distinct powder particles of a second and different metal superimposed directly on the first coat, the second metal having a solution potential higher than the solution potential of the first metal.

7. An article according to claim 6 wherein the base metal surface is of a solution potential higher than the solution potential of the first metal.

8. An article according to claim 6 wherein the first metal is selected from the group consisting of cadmium, tin, lead, copper, silver and mercury; and the second and diiferent metal is selected from the group consisting of zinc, cadmium, tin, nickel, copper, lead, silver, and alloys thereof.

9. An article according to claim 8 wherein the base metal surface is ferrous.

10. An article according to claim 6 wherein the base metal surface is ferrous, the first metal is copper, and'the second metal is zinc.

11. A method of coating metal surfaced articles comprising the steps of:

immersing the metal surfaced articles in an ionized acidic salt solution of a first metal having a solution potential lower than the solution potential of the metal surfaced articles to deposit a relatively thin crystalline layer of the first metal onto the metal surfaced articles, and then impacting the immersion deposited articles with a dynamically agitated hydraulic suspension of powder particles of a second metal in a plate promoting liquid carrier to further coat the articles with a superimposed coating of multi-layers of impacted and mechanically distorted, work hardened discrete and distinct interfitting particles, the second metal being different from the first metal and having a solution potential higher than the solution potential of the first metal.

12. The method according to claim 11 wherein the first metal is selected from the group comprising of cadmium, tin, lead, copper, silver and mercury; and the second metal is selected from the group consisting of zinc, cadmium, tin, nickel, copper,,lead, silver and alloys thereof.

13. The method according to claim 12 wherein the metal surfaced articles are ferrous.

References Cited in the file of this patent UNITED STATES PATENTS 2,251,410 Koehring Aug. 5, 1941 2,428,033 Nachtman Sept. 30, 1947 2,459,172 Luketmeyer Ian. 18, 1949 2,473,712 Kinney June 21, 1949 2,640,002 Clayton May 26, 1953 2,667,429 Diifenderfer Jan. 26, 1954 2,873,216 Schnable Feb. 10, 1959 2,892,801 Sargent June 30, 1959 2,902,748 Schaefer Sept. 8, 1959 2,916,337 Fike Dec. 8, 1959 

6. AN ARTICLE HAVING A BASE METAL SURFACE AND A PERMANENT METALLIC THEREON, THE COATING CONSISTING ESSENTIALLY OF: A RELATIVELY THIN, IONIZED SALT SOLUTION DEPOSITED CRYSTALLINE COAT OF A FIRST METAL ADHERING DIRECTLY TO THE BASE METAL SURFACE; AND A RELATIVELY THICK COAT OF BUILT UP LAYERS OF MECHANICALLY DISTORTED AND CLOSELY INTERFITTING DISCRETE AND DISTINCT POWDER PARTICLES OF A SECOND AND DIFFERENT METAL SUPERIMPOSED DIRECTLY ON THE FIRST COAT, THE SECOND METAL HAVING A SOLUTION POTENTIAL HIGHER THAN THE SOLUTION POTENTIAL OF THE FIRST METAL. 